As interest in prevention and safety against traffic accident is increasing, a research and development of an imaging device for on-vehicle monitoring system has been actively conducted. Such an imaging device is expected to be a part of a driving support system for automobiles. To this end, it is desirable to achieve a high monitoring performance to allow the device to distinguish the colors of traffic lights and lanes under a condition in which a difference or contrast between brightness and darkness is so large that the visibility is poor. Such a condition may occur when a high-luminance signal light and/or a vehicle lamp (e.g., headlight) is illuminated in darkness of night. That is, it is desirable for an imaging device to realize a wide dynamic range and color reproducibility consistent with human color vision characteristics.
In order to improve the monitoring performance of pedestrians and lanes in darkness, IR color cameras have been provided. IR color cameras use a color filter that transmits near infrared rays and performs color reproduction with sensitivity in the near infrared light region.
The IR color cameras do not have an infrared ray cut filter (IR cut filter). That is, the IR color cameras are sensitive to light in the near infrared light region. Therefore, by observing the near infrared light emitted from an observation target or by observing the reflected light of near infrared light emitted from an infrared light projector, the IR color cameras can perform imaging even in the darkness where light from a head lamp of a vehicle is not irradiated. In other words, the IR color cameras expand the image sensitive luminance range so as to expand the dynamic range thereof.
JP2014-165528A (PLT 1) teaches an imaging device that can acquire an output signal having a broad dynamic range (i.e., high sensitivity) and color reproducibility similar to human color vision characteristics. To be specific, the imaging device of PLT 1 takes an image using an IR color camera, acquires a luminance signal, which includes both visible light and near infrared light, and a color signal, which is obtained by removing only the near infrared light component from a color signal including both the visible light and the near infrared light, and combine the luminance signal and the color signal appropriately to acquire the output signal.